1. Field of the Invention
The present invention relates generally to magnetic resonance tomography (MRT) used in medicine to examine patients. The present invention relates especially to a magnetic resonance tomography device, wherein vibrations of device components especially in the low frequency range are attenuated by encapsulation of the MRT device.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been used successfully as an imaging method in medicine and biophysics for more than 15 years. With this examination method the subject is exposed to a powerful and constant magnetic field. As a result, the previously random nuclear spins of the atoms in the subject are aligned. Radio frequency energy can now stimulate these “ordered” nuclear spins to a specific vibration. This oscillation generates the actual measurement signal in MRT and this is detected by suitable receiver coils. The use of non-homogenous magnet fields, generated by gradient coils, allows the measurement object to be spatially coded in all three spatial directions. The technique allows free selection of the layer to be imaged, so sectional images of the human body can be recorded in all directions. MRT as a sectional imaging method in medical diagnostics is primarily characterized as a non-invasive examination method with a versatile contrast capability. MRT currently uses pulse sequences with a high-gradient capacity, which allow excellent imaging quality at measurement times of seconds and minutes.
The constant technical development of the components of MRT devices and the introduction of faster imaging sequences have opened up an increasing number of areas of use for MRT in medicine. Real-time imaging to assist with minimally invasive surgery, functional imaging in neurology and perfusion measurement in cardiology are just a few examples.
FIG. 8 shows a schematic section through an MRT device according to the prior art. The section shows further components of the inner area enclosed by the basic field magnet 1. The basic field magnet 1 contains superconducting magnet coils in liquid helium and is surrounded by a magnet shell 12 in the form of a twin-shell tank. The so-called cold head 15 outside the magnet shell 12 is responsible for keeping the temperature constant. In the inner area enclosed by the magnet shell 12 (also referred to as the magnet vessel) the gradient coil system 2 is suspended concentrically by means of carrier elements 7. The radio frequency resonator 13 (also called a body coil) is also inserted concentrically inside the gradient coil system 2. The radio frequency resonator 13 has the task of converting the RF pulses emitted by a power transmitter into a magnetic alternating field to stimulate the atomic nuclei of the patient 18, and then to convert the alternating field from the preceding nuclear moment into a voltage fed to the receiver link. The upper section of the radio frequency resonator 13 is connected mechanically via cladding 29 to the magnet shell 12. So-called tongues 30 are mounted in a contiguous manner on the lower section of the high-frequency resonator 13 by means of which the radio frequency resonator 13 is in turn connected mechanically via cladding 29 and by means of carrier elements 7 to the lower section of the magnet shell 12. The patient 18 is inserted into the opening or inner area of the system on a platform 19 movable on guide rails 17. The platform 19 is disposed on a vertically adjustable support frame 16.
The basic structure of the basic field magnet is shown respectively in FIG. 9. It shows the basic field magnet 1 (e.g. an axial superconducting air coil magnet with active shielding) that generates a homogenous magnetic basic field in an inner area. The superconducting magnet 1 internally has superconducting coils in liquid helium. The basic field magnet is surrounded by a twin-shell tank, generally made of high-grade steel. The inner tank containing the liquid helium, which also partly serves as a winding body for the magnet coils, is suspended by means of glass fiber reinforced plastic rods with low heat conductivity from the outer tank, which is at room temperature. There is a vacuum between the inner and outer tanks.
The cylindrical gradient coil system 2 is inserted concentrically into the inner area of the basic field magnet 1 by means of carrier elements 7. The body coil 13 is also concentrically inserted therein.
The gradient coil system 2 has three windings, which generate gradient fields spatially perpendicular to each other and proportional to the current in each winding. As shown in FIG. 10, the gradient coil system 2 includes an x-coil 3, a y-coil 4 and a z-coil 5, each of which is wound on a coil core 6, thereby generating respective gradient fields in the directions of the Cartesian coordinates x, y and z. Each of these coils is equipped with its own power supply in order to generate independent current pulses at the correct amplitude and time according to the sequence programmed in the pulse sequence controller. The currents required are around 250 A. As the gradient switching times should be as short as possible, current rise rates of around 250 kA/s are required. In an extraordinarily powerful magnet field, as is generated by the basic field magnet 1 (typically between 0.22 to 1.5 tesla), such switching processes are associated with significant mechanical vibration due to the Lorentz forces that arise. All system components linked mechanically to the gradient coil system 2 (housing, covers, basic field magnet tank or magnet shell, body coil BC, etc.) are stimulated to forced vibration.
Since the gradient coil system 2 is generally surrounded by conductive structures (e.g. high-grade steel magnet shell, conductive copper surfaces of the RF resonator), the pulsed fields trigger eddy currents therein, which interact with the basic magnet field 1 to exert forces on the aforementioned structures and also stimulate these structures to vibrate.
A further vibration source, which primarily causes the magnet vessel to vibrate, is the so-called cold head 6, which ensures that the temperature of the basic field magnet 1 is maintained. It is driven by a compressor and subjects the shell of the basic field magnet 1 to mechanical impact.
Vibration of the different MR components has a negative effect on the MR system in many ways:                1. An extremely high level of air-borne noise is generated, which is disturbing for the patient, the operating personnel and other people in the vicinity of the MR device.        
2. The vibration of the gradient coil and the basic field magnet and the transmission of said vibration to the RF resonator in the inner area of the basic field magnet or the gradient coil is manifested in inadequate clinical image quality, which can even result in misdiagnosis (e.g. with functional imaging fMRI).                3. If the vibration of the magnet shell—i.e. the outer tank—is transmitted via the glass fiber reinforced plastic rods to the inner tank or the superconductor itself is stimulated to vibrate, a higher level of helium evaporation takes place—as with an ultrasonic atomizer—thereby incurring higher costs.        
As already mentioned, most vibration or most noise originates in some way from the gradient coils (GC). The noise generated by the cold head is only 70 to 80 dB compared with 120 dB by the gradient coil, which transmits this much higher value in different ways to the magnet shell and the RF resonator.
To prevent transmission of the noise to the RF resonator or the copper eddy current surfaces representing such, various measures are known:
Firstly, the large surfaces of the copper film that are conventionally inserted relatively loosely in a carrier tube with a paper lattice structure were significantly reduced by “slots”. Secondly these films were connected rigidly and permanently to the carrier tube so that only vibration of the carrier tube could also result in vibration of the copper conductive surfaces. Thirdly, vibration of the carrier tube was impeded by significantly increasing the mass of the carrier tube using other materials.
Despite these modifications further noise transmission still occurs from the gradient coil to the RF resonator and also to the magnet shell. There are essentially three transmission mechanisms, which are outlined below:                I. Switching the gradient coil causes eddy currents to be generated both in the magnet shell and in the RF resonator and the Lorentz forces of the eddy currents as before cause vibration in the magnet shell.        II. The gradient coil and RF resonator or magnet shell and gradient coil respectively represent two cylinders, one inside the other, the radial distance of which—in the form of an air gap—between magnet and the gradient coil system is approx. 1 cm and between and the body coil only approx. 3 cm. The gradient coil stimulates the air in this air gap to vibrate and the vibration is transmitted respectively to the magnet shell and the RF resonator.        III. The gradient coil is suspended concentrically in the opening of the magnet shell by means of carrier elements. Vibration of the gradient system is transmitted to the magnet shell via this mechanical support system. The RF coil is similarly suspended inside the vibrating magnet shell. This vibration is transmitted to the RF resonator.        
In the prior art the transmission of vibration energy to the magnet shell or the RF resonator and noise emission via the magnet shell or via the RF resonator is counteracted by the use of mechanical and/or electromechanical vibration attenuators. Generally these are passive in action, e.g. rubber bearings, or piezo-actuators for example integrated in the gradient coil, which are controlled to produce counteracting vibrations, thereby reducing the vibration amplitude of the gradient coil. Vibration of the magnet shell is generally attenuated by cushions against the gradient coil.
The following passive measures are generally also used to reduce vibration:                use of thick and heavy materials        attenuation layers applied from “outside” (e.g. tar)        
It is also known that vibration damping can be achieved by inserting sound-absorbing so-called acoustic foams in the area between the carrier tube and the gradient coil.
For example in published patent application EP 1 193 507 A2 the magnet shell of the basic field magnet is coated inside and outside with an acoustically attenuating foam mass and the front face is also provided with noise-attenuating caps. Such encapsulation of the sound-inducing components of an MRT system in particular by means of an inherent shell structure is also disclosed in EP 1 077 382 A2, Patent Abstracts of Japan Vol. 1998, No. 03, 27 Feb. 1998—JP 09 299348 A and in the U.S. Pat. No. 5,084,676. The published patent application DE 198 38 390 A1 discloses an MRT device with a sound attenuation arrangement, by means of which the gradient field magnet system is encapsulated off from the patient. A similar encapsulation is disclosed in EP 0 350 640 A wherein the carrier tube holding the patient is extended axially beyond the gradient field magnet system and at the same time is expanded in a flared manner on both sides. Sound reduction of a different type is achieved according to Patent Abstracts of Japan, Vol. 2000, No. 11, 3 Jan. 2001, JP 2000 232966 A by a special gradient coil design.
Nevertheless the acoustic emission of a current standard MRT device is still very high, particularly in the low frequency (50–200 Hz) range.